Capacitance technique improves level measurement accuracy

Prophetstown, IL--Measuring liquid level inside a tank can be a critical process. If the level is mis-measured, overfilling can occur. If the liquid happens to be a hazardous chemical, disaster can result. In some cases, underfilling can be even worse: Reactive species can sudden- ly explode.

Unfortunately, traditional level measurement techniques--for liquids and solids--fall short under certain conditions. Most can achieve accuracies no better than one to two percent of actual level. Various physical phenomenon that render the measurements meaningless can compromise other methods.

By incorporating a number of new features in a capacitance-type measurement system, an engineer has erased many of the inaccuracies and compromises of level measurement systems. Gary G. Sanders, vice president of en-gineering for Penberthy, Inc., has designed a system that addresses the electronic and mechanical problems of capacitance measurement.

On the electronic side, Sanders addressed the key problems of one to two percent inaccuracy and the lack of a zero-based reference. His solution: Designing a linear bipolar current source and sink. The current source and sink acts as a bipolar model of a digital volt meter. Sanders chose that electronic configuration because, he says, "The digital volt meter is one of the most accurate instruments on the electronics test bench today."

Using this design, the system now bases all of its measurements on time. During operation, the system uses a three-step process: The current source charges the probe's capacitance to a fixed positive amplitude; then charges it to a fixed negative amplitude; then to zero. By calculating the ratio of the actual charg- ing time to a fixed time window, the device's digital circuitry determines capacitance.

On the mechanical side, Sanders addressed a separate problem. Capacitive measurement systems typical- ly employ Teflon™-based materials to seal the probe. Unfortunately, pressures inside vessels sometimes reach 2,000 psi, causing cold flow of the Teflon and leakage in the seal. Consequently, maintenance personnel must re-torque the seals on a weekly basis. Worse, those types of seals are not recognized as explosion-proof.

Sanders solved those problems by designing a pressure-activated sealing system using chevroned packing rings. When pressure is applied to the seal, it stretches and more effectively seals the opening. He also used an isolated metal blocking ring with a nonflammable, nonconductive epoxide backing to provide a flame path seal-off, thus making the unit explosion-proof.

The resulting system expands the potential uses for capacitance-based level measurement systems into areas where their application was previous- ly impossible. It vastly improves ac-curacy of readings, offers an explosion-proof rating, and, because it eliminates re-torqing of seals, reduces the time required to carry out maintenance procedures. "Customers want to install the instrument, walk away, and not worry whether it's performing accurately," Sanders says. "With this system, they can do that."

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